A technique by which a transistor is formed using a semiconductor layer formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. A silicon film is widely known as a semiconductor layer applicable to the transistor.
Whether an amorphous silicon film or a polycrystalline silicon film is used as a semiconductor layer of a transistor depends on the purpose. For example, for a transistor in a large-sized display device, an amorphous silicon film is preferably used because a technique for forming a film on a large-sized substrate has been established. On the other hand, for a transistor included in a high-performance display device where driver circuits are formed over the same substrate, a polycrystalline silicon film which can form a transistor having a high field-effect mobility is preferably used. As a method for forming a polycrystalline silicon film, high-temperature heat treatment or a laser process which is performed on an amorphous silicon film has been known.
Further, in recent years, an oxide semiconductor layer has been attracting attention. For example, a transistor including an oxide semiconductor layer which contains indium, gallium, and zinc and has a carrier density less than 1018/cm3 is disclosed (see Patent Document 1).
An oxide semiconductor layer can be formed by a sputtering method, and thus can be used for a transistor in a large-sized display device. Moreover, a transistor including an oxide semiconductor layer has a high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
It is known that a transistor including an oxide semiconductor layer has an extremely low leakage current when the transistor is off. For example, a CPU with low-power consumption utilizing low leakage current which is characteristic of the transistor including an oxide semiconductor layer is disclosed (see Patent Document 2).